A typical race gear or inner race of a constant-velocity joint fits inside a ball cage that in turn fits inside the cup-shaped outer race of the gear, with an array of coupling balls each seated in a respective outwardly open groove of the race gear and an inwardly open groove of the outer race and passing through a respective window of the cage. Such joints typically have six balls and are commonly used, for instance, to transmit torque from the engine to the front wheels of a front-drive vehicle. See U.S. Pat. No. 1,665,280.
The race gear, which acts as an inner race, is a body formed with a central normally throughgoing splined bore for connection to a shaft. Externally it has a plurality of lobes each having a radially outwardly directed outer surface that is formed as a surface section of an imaginary sphere centered on a point lying on the axis of the bore, so that it is bicurved. Between the lobes the race gear is formed with radially outwardly open generally semicircular-section race grooves whose floors are outwardly convex and centered on the same point. The outer race has the complementary radially inwardly open race grooves to hold the balls that couple the races together.
Obviously the quality of the constant-velocity joint depends largely on the accuracy with which the parts are made. Furthermore constant-velocity joints are produced in large quantities with, for instance, as many as ten being used in a four-wheel drive vehicle. They must be very reliable, have a long service life, and manufactured at the lowest possible cost.
The starting part for the race gear is cast piece of high-quality steel whose shape generally corresponds to the desired finished shape. It is normally first bored out and internally splined to form its central hole, and to allow it to be accurately positioned for subsequent machining and finishing of its race grooves and outer surfaces.
In the manufacturing method known in practice the grooves of the race gears are produced by broaching, milling, or grinding. The lobe outer surface, however, is often finished in a rotating manner. It is known from EP 0 921 329 B1 to produce both the lobe outer surface and the race grooves by means of rotating in one setting. In this method the race grooves extending axially are successively finished groove by groove, the workpiece being angularly stepped for changing from one race groove to the next. As a final step the lobe outer surface is finished.
It is further known from WO2006/058556 A1 to simultaneously process the race grooves and/or parts of the lobe outer surface of race gears using rotating tools. The rotational axes of the tools can be offset radially relative to the workpiece, and are preferably on a plane orthogonal to the workpiece axis. In order to be able to process different workpiece shapes, the angle between the tool axes can also be changed. Furthermore, the workpiece is movably held along its longitudinal axis, and can be set into rotation. Overall, five movement axes are required.